Abstract: The phenotypes of insensitive gene against acetylcholinesterase (AChE) in housefly, Musca domestica , populations from Bejing city were detected during 1998～1999. With reference to the sensitive population, 11%, 34%, 56% and 24% of the individuals of the population from Haidian 99(A), Haidian 99(B), Fengtai 99 and Chaoyang 99 zones respectively possessed the insensitive gene phenotypes of AChE to dichlorovos and 4%, 4%, 8% and 20% to methomyl. The houseflies in Beijing could be divided into four types according to the insensitive gene phenotypes of AChE against dichlorovos and methomyl: (1) phenotype Ⅰwith AChE very sensitive against both dichlorovos and methomyl, (2) phenotype Ⅱ with AChE insensitive to dichlorovos but sensitive to methomyl, (3) phenotype Ⅲ with AChE sensitive to dichlorovos but insensitive to methomyl and (4) phenotype Ⅳ with AChE very insensitive to dichlorovos and methomyl. A majority of individuals possessed phenotype Ⅰ of AChE in Beijing by now.